Now, I must begin by acknowledging that 40K is a game. And that the FOC thing does actually work, ensuring that there is some commonality in the game, based on the background.
But real armies do not look like killer lists. If you were Lord Solar Macharius on a planet and heard reports of a few squads of CSM in location A, you would be unlikely to break up an infantry formation, an armoured formation, an armoured engineer unit and an artillery group to send descrete parts to investigate the few squads of CSM.
You would, with the million tanks and countless marching boots of the IG, send whole regiments to have a look.
I know that what does happen is that (f0r instance) IG armies are explained away as detachments from the different branches and combat arms that were sent to investigate. But it would be nice to see a deployment like that of the Cadian 8th from inside the old ed of the IG Codex (yes, I do realise that it was photoshopped). Where the regimental commander is manovering companies and the company commanders are manovering platoons.
In terms of military organisation, unless those squads of veterans are sequestered to the =I= then they will actually be part of a larger formation. I realise that they will not in be part of 'x platoon' in terms of FOC, but they may well be part of 'x platoon' in terms of Chain of Command, resupply, being paid and so on.
When I use a platoon, I try to do it whole and complete; ie if the platoon is of four or five squads then it is always deployed as those four or five squads, and not eaten away at to provide points for a Valkarie (a Valkarie that that unit or formation does not usually have).
Now I realise why I appear to have multiple IG armies...